1. Field of the Invention
The present invention relates to an article of manufacture which is extremely useful as a component used in the polishing of materials, particularly semiconductor crystals and integrated circuits based on silicon wafers. In polishing, the material to be polished (the workpiece) is attached to a flat cylindrical carrier and pressed against a rotating table upon which is attached a polishing pad, generally a sheet of polymeric material. An aqueous suspension of fine particles (the slurry) is poured onto the polishing pad so as to wet the contacting surfaces. The slurry-lubricated friction between workpiece and pad results in the wearing away of surface asperities on the workpiece and the production of a smooth, featureless polished workpiece surface.
For many polishing processes, particularly those operating at high rotational speeds or pressures, inadequate slurry flow across the pad/workpiece boundary gives rise to non-uniform polishing rates, poor surface quality in the finished article and deterioration of the polishing pad due to frictional heating or plastic flow of the pad polymers. Non-uniform slurry transport is commonly cited as a primary cause of polishing rate variation, particularly in the polishing of integrated circuits, a process commonly termed planarization. In planarization polishing, the pad ideally contacts only the outer surface asperities of the integrated circuit workpiece and wears them away, resulting in a finished wafer which has virtually no surface height variation across its surface, a state known as global planarity. Global planarity is required for ensuring adequate focus in subsequent photolithographic process steps in the fabrication of the final integrated circuit.
2. Description of Related Art
In response, many prior art polishing pads have been developed with improved properties. Most improvements for high speed high pressure polishing depend on improving the resistance of the pad structure to plastic flow effects (pad glazing), and ensuring that the pad is readily permeable to slurry. Most of the pads commonly employed for planarization applications exhibit pronounced pad glazing effects, which manifest themselves as a continuous reduction in rate over time. To alleviate this problem, a number of prior art refinements, typified by U.S. Pat. No. 5,216,843 have been disclosed whose purpose is to rejuvenate, or dress, the pad by abrading its surface. This extra step adds to both the cost of the process and provides an additional source of variability. Thus an ideal planarization pad would be one which provides high and stable rates without the use of a pad dressing process. For planarization applications it is also particularly desirable to have a pad which has sufficient liquid permeability to permit slurry to be delivered from the back surface of the pad directly to the pad/workpiece interface, as described in U.S. Pat. No. 5,232,875. Thus an ideal planarization pad would have high stiffness and hardness to ensure a high degree of preferential removal of surface asperities, yielding high global planarity, would not require pad dressing and possess a good uniform slurry permeability in all directions. No prior art polishing pad fully meets these criteria.
An additional requirement for a polishing pad is that it have a precisely controlled surface shape. This derives from the requirement that the pad uniformly and fully contact the workpiece surface during polishing to effect surface removal. If the pad surface is not highly flat and regular, or if there are appreciable variations in thickness, portions of the workpiece surface will not be in contact with the pad during the polishing process. This non-contacting area will experience a resulting lower rate of removal, giving rise to nonuniformities in the shape of the resulting polished workpiece surface. This problem is greatly magnified as pad stiffness increases and compressibility decreases, as is the case for planarization polishing. Many processes for ensuring proper pad shape have been employed in prior art, such as grinding, buffing, or slicing processes. All of these processes greatly add to the complexity and cost of manufacturing polishing pads. Additionally, most of them do not yield sufficient surface shape accuracy for highly demanding applications such as planarization. For example, typical pad surfacing processes for planarization pads such as Rodel IC1000 (available from Rodel, Inc., Newark, Del.) produce a thickness variation of .about..+-.0.003 in. It would be highly desirable to reduce this to less than .+-.0.001 in. in order to improve the uniformity of the polishing process. Currently this cannot be done without significant increases in manufacturing cost resulting from the introduction of additional processing steps. Thus, an ideal polishing pad would be one wherein its surface shape and dimensions could be fixed during its initial formation, without subsequent finishing steps, i.e. it could be produced to final net shape with a minimum of manufacturing steps.
All known prior art polishing pads employed by those skilled in the art may be divided into three main classes;
1. Polymer-impregnated felts, PA1 2. Microporous elastomer films (also known as Poromerics), and PA1 3. cellular polymer foams.
The first class of pads are typified by U.S. Pat. No. 4,728,552 and related art. They are generally produced by preparing a fiber-based felt matrix, which is then impregnated with polymer, generally polyurethane. The urethane coats the fiber surfaces and bonds the fibers together into an interconnected elastic composite which has bulk porosity. The bulk porosity allows passage of fresh slurry through the body of the pad and simultaneously serves as a means of passage of workpiece debris and other dross away from the workpiece surface. Such pads are commonly manufactured by preparing a continuous roll or web of felt, impregnating the felt with polymer, curing the polymer, and generating the final pad thickness and lateral dimensions by slicing, buffing, and cutting operations. The process of manufacture is laborious, complex, and difficult to yield pads with highly precise dimensions. It does not produce pads of net shapes directly.
The three dimensional orientation of the bulk pad porosity is largely determined by the three dimensional orientation of the constituent felt fibers. As disclosed in U.S. Pat. No. 4,728,552 the fiber orientation in the felt, while generally considered to be random, is not random in all three dimensions. Generally fiber orientation is largely parallel to the major plane of the felt web. This orientation effect was deliberately exploited by the inventors to effect improvements in pad durability and polishing rate.
As disclosed in U.S. Pat. No. 4,728,552, the urethane phase of such pads is primarily responsible for the polishing activity. Thus, changes in the fraction of urethane making up the outer surface of the pad will result in variations in polishing performance. The urethane fraction at the pad surface is strongly influenced by the nature and extent of bulk porosity exposed at the outer surface of the pad. The non-random, non-isotropic nature of this porosity in impregnated felts therefore results in a high degree of intrinsic performance variability in this class of polishing pads. This variability is widely recognized in the semiconductor industry and is considered a significant impediment to further improvements to semiconductor device fabrication. A serious impediment to the employment of this class of pads in the planarization process is their generally high compressibility due to the necessary high void volume, usually greater than 50%. This low compressibility gives poor global planarity, and such pads are seldom used in the planarization process. While compressibility may be reduced by increasing the fraction of polymer used to infiltrate the felt substrate, the amount required to ensure adequate stiffness results in minimal slurry permeability. It is also extremely difficult to ensure uniform polymer permeation throughout the interior of the substrate at high loadings, resulting in increased property and performance variability.
Pads of the second class, typified by U.S. Pat. No. 4,927,432 consist of porous urethane films coated on to a base material which is often an impregnated felt pad of the first class. These porous urethane films are shown in cross-section to be composed of a series of vertically oriented closed end cylindrical pores (see SurfaceTech Review Vol. 1, no. 1). The high degree of porosity in these pads results in good slurry retention and transport during use. However, it also leads to a high degree of compressibility, making such pads unsuited for flat finishing, high pressure, or planarization applications. In addition, the closed end characteristics of the vertically oriented pores prevent liquid transport through the complete pad thickness, i.e., slurry cannot be fed from the back of the pad to the pad/workpiece interface.
Pads of the third class are typified by filled cast urethane materials such as those sold by Rodel, Inc. under the trade designations IC40, IC60, and IC1000 and blown foam materials, such as Rodel MH. These materials have bulk porosity which is randomly and uniformly distributed in all three dimensions. All known examples of such materials used commercially for polishing are closed cell foams, i.e., the volume porosity is discontinuous, with a solid barrier of polymer material between each void cell in the pad. Thus bulk slurry transport does not occur, and slurry transport characteristics of these materials are very poor. Often such pads are artificially textured with grooves or perforations to improve lateral slurry transport during polishing. In addition, such pads are very prone to pad glazing during polishing; practical use of such pads for planarization requires a regular surface abrasion, termed pad dressing, to regenerate surface texture. Without pad dressing, polishing rates of such pads are variable and undesirably low. While pads of this class are those most commonly employed in planarization polishing, the above cited deficiencies represent significant barriers to their more complete usage.
While open-cell reticulated urethane foams can be produced, as typified by products produced by E. N. Murray Co. under the Foamex trade name, these materials tend to be highly compressible, with low shear strength due to their extremely high void volume fraction, typically above 70%, making them unsuitable for possible use as polishing pads in high speed, high pressure, or planarization applications.
Other methods of producing porous polymer materials have been disclosed for purposes other than the production of polishing pads.
U.S. Pat. No. 3,763,054 discloses a means of producing microporous polyurethane sheeting by melt sintering sheets of loosely bonded particles prepared by drying films of aqueous particle dispersions. The articles prepared were made via free-sintering, i.e. pressure was not applied to assist in the particle sintering process, and film dimensions were not well controlled, the shape of the outer layer of said sintered films was not determined by contact to a mold or master surface.
U.S. Pat. No. 3,917,761 discloses a process of preparing porous sintered polyimide articles useful as oil filled bearings. The process disclosed is a variant of the lost wax process. A mixture of polyimide powder and polyformaldehyde powder were intimately mixed and pressed to a compact or green body at low temperatures (preferably 25.degree. C.) and high pressure (&gt;10,000 psi). This green body was then free-sintered at a temperature well above the melting point of the lower melting polyformaldehyde phase. This caused thermal decomposition of the polyformaldehyde to formaldehyde vapor while the polyimide phase simultaneously melt sintered. The resultant structure was a microporous sintered polyimide article. A major disadvantage of such a process is the evolution of formaldehyde gas, which is now recognized as a carcinogen.
U.S. Pat. No. 4,256,845 discloses a method for manufacturing a porous thermoplastic sheet by gelling an aqueous latex dispersion containing an additional material of a preselected particle size and forming the dispersion into a sheet. This sheet is then free-sintered at a temperature at or above the melt point of the thermoplastic to form the final product from which the additional material is extracted.
U.S. Pat. No. 4,880,843 discloses a similar process for preparing a porous molded composite article of ultra high molecular weight polyethylene combined with a polyethylene wax. The mixture of powders is put into a press mold of final dimensions using a low pressure sufficient only to prevent deformation. The mold and powder is then melt sintered at a temperature in excess of the melting point of the polymer.
In light of the above information, the most desirable polishing pad for high temperature, high pressure or planarization polishing applications would be one which has a high volume of polymer material to ensure low compressibility, high stiffness, and resistance to shear forces, a high and uniform degree of permeability to slurry in all directions and has minimal pad glazing so that pad dressing is not required. It would also be particularly desirable for such pads to be produced by a process which yielded a pad of final shape and dimensions, thus reducing the number of manufacturing steps and improving the dimensional precision of the pad, with corresponding improvements in cost of manufacture and polishing quality.
Accordingly, it is the object of the present invention to provide a process for manufacturing polymer-based pads useful for polishing objects, particularly integrated circuits, which have interconnected porosity which is uniform in all directions so as to provide free and unimpeded transport of slurry through the body of the pad.
It is also the object of the present invention to provide a process which produces pads having said bulk porosity in a form which produces high and sustained polishing rates without pad dressing, said dressing step being rendered optional.
It is a further object of the invention to provide a process which produces said pads directly to final shape and dimension directly from component polymer starting materials with resorting to subsequent shaping operations such as cutting, grinding, or shaping.